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Chapter 3
Section 1 The Atom: From
Philosophical Idea to Scientific Theory
Lesson Starter
• Young people should not smoke.
• Smoking at an early age may make it more difficult to
quit smoking later.
• Which of the above statements is an opinion and
which is a theory?
• Which is similar to Aristotle’s statements?
Chapter 3
Section 1 The Atom: From
Philosophical Idea to Scientific Theory
Objectives
• Explain the law of conservation of mass, the law of
definite proportions, and the law of multiple
proportions.
• Summarize the five essential points of Dalton’s
atomic theory.
• Explain the relationship between Dalton’s atomic
theory and the law of conservation of mass, the law of
definite proportions, and the law of multiple
proportions.
Chapter 3
Section 1 The Atom: From
Philosophical Idea to Scientific Theory
Foundations of Atomic Theory
• The transformation of a substance or substances
into one or more new substances is known as a
chemical reaction.
• Law of conservation of mass: mass is neither
created nor destroyed during ordinary chemical
reactions or physical changes
Chapter 3
Section 1 The Atom: From
Philosophical Idea to Scientific Theory
Chemical Reaction
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Chapter 3
Section 1 The Atom: From
Philosophical Idea to Scientific Theory
Law of Conservation of Mass
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Chapter 3
Section 1 The Atom: From
Philosophical Idea to Scientific Theory
Foundations of Atomic Theory, continued
• Law of definite proportions: a chemical compound
contains the same elements in exactly the same
proportions by mass regardless of the size of the
sample or source of the compound
• Law of multiple proportions: if two or more different
compounds are composed of the same two elements,
then the ratio of the masses of the second element
combined with a certain mass of the first element is
always a ratio of small whole numbers
Chapter 3
Section 1 The Atom: From
Philosophical Idea to Scientific Theory
Law of Definite Proportions
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Chapter 3
Section 1 The Atom: From
Philosophical Idea to Scientific Theory
Law of Multiple Proportions
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Chapter 3
Section 1 The Atom: From
Philosophical Idea to Scientific Theory
Law of
Conservation of
Mass
Chapter 3
Section 1 The Atom: From
Philosophical Idea to Scientific Theory
Law of Multiple Proportions
Chapter 3
Section 1 The Atom: From
Philosophical Idea to Scientific Theory
Dalton’s Atomic Theory
• All matter is composed of extremely small particles
called atoms.
• Atoms of a given element are identical in size, mass,
and other properties; atoms of different elements
differ in size, mass, and other properties.
• Atoms cannot be subdivided, created, or destroyed.
Chapter 3
Section 1 The Atom: From
Philosophical Idea to Scientific Theory
Dalton’s Atomic Theory, continued
• Atoms of different elements combine in simple wholenumber ratios to form chemical compounds.
• In chemical reactions, atoms are combined,
separated, or rearranged.
Chapter 3
Section 1 The Atom: From
Philosophical Idea to Scientific Theory
Modern Atomic Theory
• Not all aspects of Dalton’s atomic theory have proven
to be correct. We now know that:
• Atoms are divisible into even smaller particles.
• A given element can have atoms with different
masses.
• Some important concepts remain unchanged.
• All matter is composed of atoms.
• Atoms of any one element differ in properties from
atoms of another element.
Chapter 3
Section 2 The Structure of the Atom
Lesson Starter
• Even though the two shapes look different, the
characteristics of the various parts that compose
them are the same.
• The same is true with the atom.
• Though atoms of different elements display different
properties, isolated subatomic particles have the
same properties.
Chapter 3
Section 2 The Structure of the Atom
Objectives
•
Summarize the observed properties of cathode
rays that led to the discovery of the electron.
•
Summarize the experiment carried out by
Rutherford and his co-workers that led to the
discovery of the nucleus.
•
List the properties of protons, neutrons, and
electrons.
•
Define atom.
Chapter 3
Section 2 The Structure of the Atom
The Structure of the Atom
• An atom is the smallest particle of an element that
retains the chemical properties of that element.
• The nucleus is a very small region located at the
center of an atom.
• The nucleus is made up of at least one positively
charged particle called a proton and usually one or
more neutral particles called neutrons.
Chapter 3
Section 2 The Structure of the Atom
The Structure of the Atom, continued
• Surrounding the nucleus is a region occupied by
negatively charged particles called electrons.
• Protons, neutrons, and electrons are often referred to
as subatomic particles.
Chapter 3
Section 2 The Structure of the Atom
Atom
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Chapter 3
Section 2 The Structure of the Atom
Parts of the Atom
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Chapter 3
Section 2 The Structure of the Atom
Properties of Subatomic Particles
Chapter 3
Section 2 The Structure of the Atom
Discovery of the Electron
Cathode Rays and Electrons
• Experiments in the late 1800s showed that cathode
rays were composed of negatively charged particles.
• These particles were named electrons.
Chapter 3
Section 2 The Structure of the Atom
Discovery of the Electron, continued
Charge and Mass of the Electron
• Joseph John Thomson’s cathode-ray tube
experiments measured the charge-to-mass ratio of
an electron.
• Robert A. Millikan’s oil drop experiment measured the
charge of an electron.
• With this information, scientists were able to
determine the mass of an electron.
Chapter 3
Section 2 The Structure of the Atom
Discovery of the Electron, continued
Chapter 3
Section 2 The Structure of the Atom
Thomson’s Cathode-Ray Tube Experiment
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Chapter 3
Section 2 The Structure of the Atom
Millikan’s Oil Drop Experiment
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Chapter 3
Section 2 The Structure of the Atom
Discovery of the Atomic Nucleus
• More detail of the atom’s structure was provided in
1911 by Ernest Rutherford and his associates Hans
Geiger and Ernest Marsden.
• The results of their gold foil experiment led to the
discovery of a very densely packed bundle of matter
with a positive electric charge.
• Rutherford called this positive bundle of matter the
nucleus.
Chapter 3
Section 2 The Structure of the Atom
Gold Foil Experiment
Chapter 3
Section 2 The Structure of the Atom
Gold Foil Experiment on the Atomic Level
Chapter 3
Section 2 The Structure of the Atom
Rutherford’s Gold Foil Experiment
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Chapter 3
Section 2 The Structure of the Atom
Composition of the Atomic Nucleus
• Except for the nucleus of the simplest type of
hydrogen atom, all atomic nuclei are made of
protons and neutrons.
• A proton has a positive charge equal in magnitude
to the negative charge of an electron.
• Atoms are electrically neutral because they contain
equal numbers of protons and electrons.
• A neutron is electrically neutral.
Chapter 3
Section 2 The Structure of the Atom
Composition of the Atomic Nucleus,
continued
• The nuclei of atoms of different elements differ in their
number of protons and therefore in the amount of
positive charge they possess.
• Thus, the number of protons determines that atom’s
identity.
Chapter 3
Section 2 The Structure of the Atom
Composition of the Atomic Nucleus,
continued
Forces in the Nucleus
• When two protons are extremely close to each other,
there is a strong attraction between them.
• A similar attraction exists when neutrons are very
close to each other or when protons and neutrons are
very close together.
• The short-range proton-neutron, proton-proton, and
neutron-neutron forces that hold the nuclear particles
together are referred to as nuclear forces.
Chapter 3
Section 2 The Structure of the Atom
The Sizes of Atoms
• The radius of an atom is the distance from the center
of the nucleus to the outer portion of its electron cloud.
• Because atomic radii are so small, they are expressed
using a unit that is more convenient for the sizes of
atoms.
• This unit is the picometer, pm.
Chapter 3
Section 3 Counting Atoms
Lesson Starter
• Imagine that your semester grade depends 60% on
exam scores and 40% on laboratory explorations.
• Your exam scores would count more heavily toward
your final grade.
• In this section, you will learn that the atomic mass of
an element is a weighted average of the masses of
the naturally occurring isotopes of that element.
Chapter 3
Section 3 Counting Atoms
Objectives
• Explain what isotopes are.
• Define atomic number and mass number, and
describe how they apply to isotopes.
• Given the identity of a nuclide, determine its number
of protons, neutrons, and electrons.
Chapter 3
Section 3 Counting Atoms
Atomic Number
• Atoms of different elements have different numbers of
protons.
• Atoms of the same element all have the same number
of protons.
• The atomic number (Z) of an element is the number
of protons of each atom of that element.
Chapter 3
Visual Concepts
Atomic Number
Chapter 3
Section 3 Counting Atoms
Isotopes
• Isotopes are atoms of the same element that have
different masses.
• The isotopes of a particular element all have the same
number of protons and electrons but different numbers
of neutrons.
• Most of the elements consist of mixtures of isotopes.
Chapter 3
Section 3 Counting Atoms
Mass Number
• The mass number is the total number of protons
and neutrons that make up the nucleus of an
isotope.
Chapter 3
Mass Number
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Chapter 3
Section 3 Counting Atoms
Designating Isotopes
• Hyphen notation: The mass number is written with a
hyphen after the name of the element.
• uranium-235
• Nuclear symbol: The superscript indicates the mass
number and the subscript indicates the atomic
number.
Chapter 3
Section 3 Counting Atoms
Designating Isotopes, continued
•
The number of neutrons is found by subtracting the
atomic number from the mass number.
mass number − atomic number = number of neutrons
235 (protons + neutrons) − 92 protons = 143 neutrons
•
Nuclide is a general term for a specific isotope of an
element.
Chapter 3
Section 3 Counting Atoms
Designating Isotopes, continued
Sample Problem A
How many protons, electrons, and neutrons are there in
an atom of chlorine-37?
Chapter 3
Section 3 Counting Atoms
Designating Isotopes, continued
Sample Problem A Solution
Given: name and mass number of chlorine-37
Unknown: numbers of protons, electrons, and
neutrons
Solution:
atomic number = number of protons = number of
electrons
mass number = number of neutrons + number of
protons
Chapter 3
Section 3 Counting Atoms
Designating Isotopes, continued
Sample Problem A Solution, continued
mass number of chlorine-37 − atomic number of
chlorine = number of neutrons in chlorine-37
mass number − atomic number = 37 (protons plus
neutrons) − 17 protons = 20 neutrons
An atom of chlorine-37 is made up of 17 electrons, 17
protons, and 20 neutrons.
Chapter 3
Section 3 Counting Atoms
Relative Atomic Masses
• The standard used by scientists to compare units of
atomic mass is the carbon-12 atom, which has been
arbitrarily assigned a mass of exactly 12 atomic mass
units, or 12 amu.
• One atomic mass unit, or 1 amu, is exactly 1/12 the
mass of a carbon-12 atom.
• The atomic mass of any atom is determined by
comparing it with the mass of the carbon-12 atom.
Chapter 3
Section 3 Counting Atoms
Average Atomic Masses of Elements
• Average atomic mass is the weighted average of the
atomic masses of the naturally occurring isotopes of
an element.
Calculating Average Atomic Mass
• The average atomic mass of an element depends on
both the mass and the relative abundance of each of
the element’s isotopes.
Chapter 3
Section 3 Counting Atoms
Average Atomic Masses of Elements,
continued
Calculating Average Atomic Mass, continued
• Copper consists of 69.15% copper-63, which
has an atomic mass of 62.929 601 amu, and
30.85% copper-65, which has an atomic mass of
64.927 794 amu.
• The average atomic mass of copper can be
calculated by multiplying the atomic mass of each
isotope by its relative abundance (expressed in
decimal form) and adding the results.
Chapter 3
Section 3 Counting Atoms
Average Atomic Masses of Elements,
continued
Calculating Average Atomic Mass, continued
• (0.6915 × 62.929 601 amu) + (0.3085 ×
64.927 794 amu) = 63.55 amu
• The calculated average atomic mass of naturally
occurring copper is 63.55 amu.
Chapter 3
Section 3 Counting Atoms
Average Atomic Mass
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